An engine vehicle, for example, has a battery for a starter motor for starting an engine (internal combustion engine) of this vehicle. A battery deteriorates over time, and is likely to cause start failure of an engine due to deterioration. In view of this, an automobile may include a system which measures a degree of deterioration, and encourages a user to replace the battery, for example, when determining that the deterioration has proceeded to a certain extent. It is known that whether a battery has deteriorated or not can be determined based on an internal resistance of a battery.
For example, JP 2008-122165 A describes a method of detecting a degree of deterioration. In the method, a weight is multiplied to a deterioration degree SOH, which is acquired based on an internal resistance of a battery, to calculate SOH (State Of Health) of the battery. The weight to be multiplied to the internal resistance is set larger, as the battery comes toward the end of its life and the internal resistance increases. With this, the deterioration degree in accordance with the change in the internal resistance depending on the operating time of the battery can be detected.